Optical measuring systems for measuring the concentration of a gas component in a measured gas, based on wavelength modulation spectroscopy, are known from the prior art in a wide variety of embodiments, as are a multitude of different methods for operating such an optical measuring system.
In tunable laser absorption spectroscopy (TLAS), and in particular wavelength modulation spectroscopy (WMS), the wavelength modulation amplitude generally plays an important role. This is defined by way of setting of the current modulation amplitude during calibration for the operating point. However, changes to the operating point or long-term changes in the laser cause the wavelength modulation amplitude to change (despite the current modulation amplitude remaining the same). As a consequence, the change in the wavelength modulation amplitude also causes the sensor calibration to deviate from the specification limits, which then often necessitates re-calibration of the optical measuring system.
In the optical measuring systems of the type in question, in the case of changes to the operating mode or long-term changes in the laser light source compared to the time of calibration, for the purpose of stabilization of the sensor accuracy, it is known to re-set the wavelength modulation amplitude during operation by adapting the intensity of the modulation current for the laser light source, which is to say the current modulation amplitude, so that the wavelength modulation amplitude once again at least approximately corresponds to the time of calibration. Reference is made in this regard to document EP 2 610 608 B1, by way of example.
Document EP 2 610 608 B1 discloses a gas measuring device for measuring a target gas and a method for setting the width of a wavelength modulation of the gas measuring device, comprising a light source and a detection unit, by oscillating a a laser light wavelength by way of the source, so as to have a central wavelength determined by a main current, and so as to be modulated according to a modulation current at an oscillation frequency, and with a wavelength modulation width, with the central wavelength being varied by changing the main current in a longer cycle than that of the modulation current, whereby the detection unit outputs a signal according to an intensity of the laser light transmitted through a standard gas. The method furthermore comprises a step of obtaining a detection signal by detecting the laser light transmitted through the standard sample, with the central wavelength being varied, the step of obtaining a specific frequency component of the detection signal that is oscillated at a frequency that is a positive integral multiple of an oscillation frequency of the modulation current, and a step of calculating a ratio of a magnitude of a local minimum of the specific frequency component with respect to the central wavelength of the laser light and a magnitude of a local maximum of the specific frequency component with respect to the central wavelength of the laser light, and a step of setting the width of the wavelength modulation of the laser light so that the ratio satisfies a predetermined condition. This is a condition where the ratio that corresponds to the width of the wavelength modulation equals a predetermined target value on one-to-one basis. During the setting of the modulation width of the laser light, the width of the wavelength modulation is set by adapting an intensity of the modulation current.